In recent years, from social demands for low-fuel consumption and low exhaust emission, there has been an increased attention to an electric vehicle or a hybrid vehicle equipped with an alternate-current (AC) motor to run. For example, in a hybrid vehicle, an AC motor is connected to a direct-current (DC) power source such as a rechargeable battery unit through a power converter such as an inverter. The inverter converts a DC voltage supplied from the DC power source to an AC voltage and drives the AC motor with the AC voltage.
JP-A-2008-86139 corresponding to US 2008/0079385 discloses a “one-phase control” technique for reducing the number of current sensors used in a control apparatus for an AC motor mounted on an electric vehicle and a hybrid vehicle, thereby reducing the size of the apparatus near output terminals of the inverter and the cost of a control system of the AC motor. In the one-phase control technique disclosed in JP-A-2008-86139, a current sensor for detecting a phase current is provided to only one phase of the AC motor, and energization of the AC motor is controlled based on feedback of a current estimation value which is estimated based on the phase current detected by the current sensor.
By the way, a “discharge” technique to ensure safety when a vehicle is stopped has been known. In the discharge technique, a smoothing capacitor which is connected in parallel to a DC power source at an input side of an inverter is discharged after the smoothing capacitor is disconnected from the DC power source. For example, in a discharge technique disclosed in JP-3289567, after a permanent-magnet motor is stopped, non-zero current is supplied to only a d-axis which does not contribute to torque so that a q-axis which contributes to torque can be supplied with substantially no current. Thus, a smoothing capacitor is discharged without generating torque which rotates the motor.